The process of burning batch-loaded wood in ambient air at atmospheric conditions begins with the application of sufficient heat (greater than approximately 350° F. (177° C.)) to initiate a self-sustaining combustion process. Heating first causes moisture contained in the fuel to evaporate into the space in the immediate vicinity of where the fuel heating is taking place with subsequent dispersion into the atmosphere. As fuel moisture is depleted in the area of the fuel being heated, the organic components of the fuel, consisting of but not limited to such compounds as lignin, hemicellulose, and cellulose, begin to break down by way of a thermal process called pyrolysis. Pyrolysis includes both oxidation and reduction reactions initiated by the increasing temperature of the fuel. Virtually all of the formed and reformed chemical species produced by the pyrolysis process are organic species ranging from simple methane and formaldehyde to complex molecules such as benzo-a-pyrene and some notorious toxins like dioxins.
At the temperatures at which wood pyrolysis reactions take place (i.e., generally above 300° F. (149° C.)) virtually all of the pyrolysis reaction products leave a burning piece of wood in a gaseous phase. This means that, at atmospheric conditions, the pyrolysis products will migrate or disperse out of and away from the wood fuel being heated. As these gases, all of which are combustible, leave the surface of the fuel they mix with air and it's 20.9% oxygen content. At the mixing point where there are combustible gases within the range of flammability concentrations and there is adequate temperature, generally above 600° F. (316° C.), the pyrolysis product and air mixture will generate a self-sustaining combustion process usually observed as flaming.
If the pyrolysis-product gases are too rich, become too diluted by air, or there is inadequate temperature to initiate a self-sustaining combustion process the pyrolysis-product gases will not “burn” and they will leave the combustion zone either as gaseous pollutants (primarily carbon monoxide and methane) or as suspended condensate droplets or aerosols, and particulates, which make up what is generally referred to as smoke or solid particulate emissions. These incompletely combusted liquids and particles deposit on chimney walls forming creosote, which creates chimney fire hazards and clogs chimney flow paths, and, as they exit the chimney, are released as pollutants. Such pollutants can cause significant local air pollution concerns. Indeed, some local building codes prohibit new wood burning fireplaces and wood stoves due to the smoke/particulate pollution.
If excessive dilution takes place in the combustion zone, the concentration of those pyrolysis-product compounds that typically produce visible smoke in flue gases can be reduced to levels below their condensation vapor pressures. When this occurs, little or no smoke is observed in the flue gases, but the total mass of emitted incompletely burned materials remains in the flue gases.
Since the elemental makeup of wood consists primarily of carbon, hydrogen, and oxygen, the complete combustion products of wood and its pyrolysis products results primarily in carbon dioxide and water. Small amounts of nitrogen and sulfur are present in wood at tenth of a percent levels which form nitrous oxides and sulfur oxides respectively when wood is burned. Other inorganic constituents of wood include the salts of calcium, sodium, potassium, magnesium, iron, silicon, chlorine, and phosphorus, which comprise virtually the total make up of the ash materials left after complete wood combustion has taken place.
To accomplish the compete combustion of wood it would first be necessary to heat the fuel evenly throughout and then as the various species of gaseous pyrolysis products are produced they would be evenly mixed with the appropriate amounts of air for ideal combustion and then evenly heated further to the appropriate temperature for initiating combustion (i.e., ignition temperature). This complete or ideal combustion process requires an ideal set of conditions that do not occur under the natural conditions found in fireplace combustion chambers. Under normal and typical fireplace conditions pieces of wood are heated unevenly with some areas reaching temperatures adequate to initiate pyrolysis but not hot enough or uniform enough to generate enough combustible gas to initiate combustion. Because fuel heating in a fireplace is so uneven throughout the burning of a fuel load, there will always be zones, like near where flaming is occurring, where temperatures are hot enough to cause the production of pyrolysis products but not hot enough to cause them to burn or they become too dilute by mixing with air to burn. In either case, these products of incomplete combustion (PICs) escape the combustion zone and, if there are no further steps taken to combust these materials, they become pollutants discharged to the atmosphere or accumulate on chimney surfaces. The same or similar processes hold true for combustion of other woody vegetation and parts thereof, e.g. biomass and agricultural waste such as nut shells and the stalks and husks remaining after grain threshing. In the context of this Application, therefore, reference to “fireplaces”, “wood stoves” and other “wood burning apparatuses” includes apparatuses that burn wood, woody plants, biomass and agricultural waste.
Thus, there is a need for method and apparatus to reduce or eliminate PICs and other organic pollutants from wood burning apparatuses. Presently known art attempts to address this problem, but has not completely solved the problem. The following represents a list of known related art:
Reference:Issued to:Date of Issue:U.S. Pat. No. 6,237,587Sparling et alMay 29, 2001U.S. Pat. No. 5,499,622WoodsMar. 19, 1996U.S. Pat. No. 6,227,194Barudi et alMay 8, 2001U.S. Pat. No. 4,249,509SymeFeb. 10, 1981U.S. Pat. No. 3,496,890La RueFeb. 24, 1970U.S. Pat. No. 4,422,437HirscheyDec. 27, 1983U.S. Pat. No. 4,476,852Lee et alOct. 16, 1984U.S. Pat. No. 5,944,025Cook et alAug. 31, 1999U.S. Pat. No. 4,385,032Fratzer et alMay 24, 1983U.S. Pat. No. 3,468,634PaulettaSep. 23, 1969U.S. Pat. No. 5,460,511GrahnOct. 24, 1995
The teachings of each of the above-listed citations (which does not itself incorporate essential material by reference) are herein incorporated by reference. None of the above inventions and patents, taken either singularly or in combination, is seen to describe the instant invention as claimed.
U.S. Pat. No. 5,499,622 to Woods teaches a simple heating element and flue/draft enhancer to combust some PICs in a fireplace flue. Woods teaches temperatures only in the range 1100° F. up to 1500° F. (593° C. up to 816° C.), and specifically includes provisions to shut down the process if temperatures exceed 1500° F. (815° C.). Woods does not teach the use of catalysts as a secondary oxidizer. Woods requires the use of a tortuous pathway to remove particulates prior to reaching the heating elements. From 50% to 90% of the liquid condensation particles from wood combustion processes are less than 1.0 μm in diameter. Particles of this size do not “settle” out of air and are even difficult to separate from air using high velocity centrifugal forces. This fact alone renders Woods' “tortuous pathway” ineffective.
U.S. Pat. No. 4,476,852 to Lee, et al, teaches a simple catalytic insert into a fireplace flue. The catalyst element design of Lee would rapidly foul in the exhaust flue of a wood burning apparatus such as a fireplace or wood stove as it has no heating element to raise temperatures above 1500° F. (816° C.), which is needed to initiate catalytic action on the surfaces to completely oxidize PICs that may otherwise accumulate on the surfaces.
U.S. Pat. No. 3,468,634 to Pauletta teaches an incinerator for burning off unspecified “obnoxious fumes” exhausting from industrial processes. The only treatment process contemplated is heating the fumes to 800° F. to 1500° F. (427° C. to 816° C.). The afterburner includes a counter-flow heat exchanger which pre-heats incoming exhaust gases and an unspecified catalyst. The afterburner is positioned in the inlet plenum of the device, as is the catalyst. Pauletta does not address PICs, and does not contemplate using temperatures below 800° F. (427° C.) or above 1500° F. (816° C.). The heat exchanger design of Pauletta would rapidly foul if utilized in the exhaust flue of a wood burning apparatus such as a fireplace or wood stove.
U.S. Pat. No. 5,460,511 to Grahn teaches an afterburner for oxidizing the byproducts of incomplete combustion from internal combustion engines and wood stoves. The exhaust gases pass through a counterflow heat exchanger before entering the “firebox”, which is located in the outlet plenum of the heat exchanger. Grahn contemplates the use of a catalyst-coated steel screen inside the firebox in order to reduce the required temperature for oxidizing the exhaust gases. The heat exchanger design of Grahn would rapidly foul if utilized in the exhaust flue of a wood burning apparatus such as a fireplace or wood stove. Grahn does not contemplate using temperatures above 600° F. to 800° F. (316° C. to 427° C.), and as described never exceeded 580° F. (304° C.). Moreover, Grahn relies on a self-sustaining reaction within the afterburner to function. Grahn's heating element and the heat generated by catalysis are used to initiate self-sustaining re-combustion, with the addition of an external air source, but the heating elements do not maintain temperatures above 1500° F. (816° C.) in order to completely oxidize PICs and prevent buildup on the surfaces of the system.
U.S. Pat. No. 4,385,032 to Fratzer, et al, teaches a catalytic waste gas converter for internal combustion engines using catalyst coated steel screens of various geometries to maximize surface area contact with the gas streams. Fratzer does not contemplate the use of an afterburner heat source, nor does Fratzer contemplate temperatures of greater than 1500° F. (816° C.). Nor does Fratzer contemplate use in a sooty exhaust stream, and in fact Fratzer would rapidly foul and become unusable if installed in the exhaust flue of a chimney or wood burning stove. Fratzer actually relies on a mechanical filter to remove particulate carbon waste (i.e. soot) before it reaches the catalytic gas converter so as to not clog the converter.
U.S. Pat. No. 5,944,025 to Cook et al., teaches a design for a reduced-smoke cigarette. Hot gases from a combustion chamber are passed through a catalyst coated ceramic mullite honeycomb before passing through a section containing cut tobacco and delivered to the smoker. The catalytic section reduces carbon monoxide from the combustion gases. The only fuels taught for the combustion chamber are clean burning liquid or semi-solids, which produce essentially no particulates. Cook does not teach methods for reducing PICs from wood burning systems, does not teach methods of reducing soot, and requires forced air flow (produced by the smoker inhaling) to treat gases. Cook does not teach the use of a separate heat source to completely oxidize PICs.
As seen by these references, the existing art relies on either heating exhaust streams to approximately 900° F. (480° C.) up to approximately 1472° F. (800° C.), or provision of a catalytic surface, or both, to reduce undesired emissions. The inventor has found this to be at best only a partial solution. With temperatures limited to below 1500° F. (816° C.) the PICs exhausted from a wood burning apparatus will not be completely oxidized, unless an oxidizer agent is added to the exhaust stream prior to heat treatment. Carbon particulates, in the form of soot, will not be completely oxidized, such that particulate emissions will still be problematic and internal surfaces in the flue and treatment apparatus will become fouled. The addition of catalysts into the exhaust stream aids in reducing undesired gaseous emissions, especially carbon monoxide and volatile HCs, but do not solve the problem of particulate emissions and will lose effectiveness over time as particulates become deposited on the catalyst bed surfaces.
Wood burning apparatus for home use vary considerably in size and design and performance, but typically home fireplaces produce approximately 50 to 150 cfm of exhaust flow during operation, while home wood stoves typically produce approximately 20 cfm of exhaust flow. The exhaust stream temperatures at the outlet of the firebox, where the woody material is burned, range from 300° F. to 600° F. (150° C. to 316° C.) for home fireplaces and wood stoves. Commercial wood burning apparatuses, such as boiler systems using agricultural waste or lumber mill waste, tend to have higher, steadier exhaust flow rates with more uniform smoke densities.
Wood stove exhaust streams tend to have lower flowrates and higher discharge temperatures, with higher smoke density—meaning higher density of PICs, carbon particles, and combustion gasses—than fireplaces. Fireplace exhaust streams tend to be diluted significantly and variable. Variability is caused by differing fireplace and flue designs, differences in maintenance/cleaning, use of a variety of types of logs and variable moisture levels, weather conditions at the flue outlet, and other conditions. The higher smoke density of wood stove exhaust streams means that treatment systems using catalysts are often able to maintain temperatures on the surface of the catalyst bed at or above 800° F., but using a catalyst bed alone for a fireplace is generally not reliable for creating a self-sustaining reaction to maintain temperatures above 800° F. on the catalyst surface. However, even for wood stoves, while catalyst beds create high temperatures on their surfaces, where they react with organic pollutants, they are not able to heat the actual exhaust stream itself to high enough temperatures to eliminate PICs. Only the particles which come in direct contact with the catalyst surface are catalyzed or heated sufficiently to oxidize.
Using high temperatures (>1500° F.) to reduce PICs is reliable, but it may be desirable to reduce temperatures to reduce power requirements and/or to enable use of less expensive materials. Injecting strong oxidizer agents into the exhaust stream prior to heat treatment can significantly reduce the temperature required to remove PICs, while the heat treatment process itself causes decomposition of excess oxidizer agent.
None of the foregoing references teach apparatus and methods for directing wood burning apparatus exhaust streams into a reaction chamber having multiple reaction chamber channels with heating elements for raising exhaust stream temperatures greater than 800° F. (427° C.). None of the foregoing references teach apparatus or methods for injecting oxidizing agents into the exhaust stream from a wood burning apparatus prior to entering a heated reaction chamber. None of the foregoing references teach apparatus or methods for maintaining lower temperatures for treatment of exhaust streams when injecting oxidizing agents into the exhaust stream and higher temperatures when no injection is used.